One problem with remediation programs for problem aggression may be that they are begun too late, after aggressive styles have become highly stable elements of personality, resistant to change. This research attempts to lay the ground work for earlier prevention and intervention by studying the origins of highly aggressive behavior, and the factors that contribute to the development of aggression and other emotional or behavior problems. Children are studied at one year of age, and followed up at school-age. Two main findings have emerged: (1) There is evidence for an aggressive syndrome in young boys, marked by sensitivity to environmental stress, high intensity of aggression, labile emotions, and high stability of aggression over time and in multiple settings. (2) Anger or conflict between adults act as a powerful instigator of aggression and other emotional and behavioral problems. Responding to background anger is stable from an early age, but is modifiable by experience, with repeated exposure resulting in increased sensitization and greater aggressiveness. Emotional reactivity to background anger increases with age, but so does the range and flexibility of coping patterns.